Heretofore, electrolytic electrodes using substrates of valve metals such as titanium (Ti) have found recognition as outstanding insoluble metal electrodes and have found utility as such in various fields of electrochemistry. Particularly in the industry specializing in electrolysis of common salt, these electrodes have been found extremely useful as anodes for the generation of chlorine. As valve metals, tantalum (Ta), niobium (Nb), zirconium (Zr), hafnium (Hf), vanadium (V), molybdenum (Mo), tungsten (W), etc. have been known to the art besides Ti mentioned above.
These metal electrodes are generally obtained by coating substrates of the metal Ti with various electrochemically active substances represented by platinum-group metals or oxides thereof. Such electrodes disclosed by U.S. Pat. No. 3,632,498 and U.S. Pat. No. 3,711,385 are familiar examples. These electrodes, particularly when used for the generation of chlorine, are capable of retaining a low chlorine overvoltage for a long time.
When such a metal electrode as described above is adopted as an anode in electrolysis intended for or entailing generation of oxygen, the overvoltage of the anode is gradually raised. In an extreme case, this rise of overvoltage may induce a severe problem in that the anode will be passivated and prevented from continuing electroysis any further. This phenomenon of the passivation of the electrode is believed to be best explained by a postulate that the Ti substrate is oxidized by the oxygen issuing from the oxide coat itself of the electrode or by the reaction of the substrate with the oxygen or the electrolyte permeating the coat and reaching the substrate. Consequently, a non-conducting Ti oxide coating forms on the substrate. Also, since the non-conducting oxide is formed in the interface between the substrate and the coat of the electrode, a further disadvantage may arise in that the oxide interface possibly could cause the electrode coat to separate from the substrate and eventually render the electrode completely unserviceable.
Electrolytic processes in which the anode product is oxygen, or in which oxygen is generated at the anode as a side reaction, include: (1) electrolysis using a sulfuric acid bath, a nitric acid bath, alkali baths, or the like; (2) electrolytic separation of Cr, Cu, Zn, or the like; (3) various forms of electroplating; (4) electrolysis of dilute brackish water, brine water, hydrochloric acid, or the like; and (5) electrolysis for the production of chlorates, and so forth.
To date, however, the problem mentioned above has been a serious obstacle to the effective use of metal electrodes in these industrial fields.
As a solution to this problem, a technique of preventing the electrode from being passivated due to the permeation of oxygen is described in U.S. Pat. No. 3,775,284. This technique involves interposing between the conducting substrate and the coat of the electrode a barrier layer formed of a Pt-Ir alloy or an oxide of cobalt (Co), manganese (Mn), palladium (Pd), lead (Pb), or platinum (Pt). The substances which constitute the interposed barrier layer, to some extent, prevent oxygen from being dispersed into the substrate during electrolysis. Nevertheless, the substances of the barrier layer possess a fair degree of electrochemical activity and, therefore, react with the electrolyte permeating the coat of the electrode and produce electrolytic products, e.g., gas, on the surface of the interposed barrier layer. Thus, there ensues the possibility that the physical and chemical actions of the electrolytic product will impair the tight adhesion of the coat of the electrode to the substrate and cause separation of the coat of the electrode from the substrate before the service life of the substance constituting the coat of the electrode is exhausted. Additionally, the barrier layer itself causes problems in that it prevents the electrode from being sufficiently corrosionproof. Thus, the solution produces a new problem and fails to provide lasting protection for the electrode.
U.S. Pat. No. 3,773,555 discloses an electrode which is coated with a laminate composed of a layer of an oxide, such as of Ti, and a layer of a platinum-group metal or an oxide thereof. This electrode nevertheless has the disadvantage in that the electrode undergoes passivation when used in electrolysis in which oxygen is liberated.